1. Field of the Invention
The present invention relates generally to a lithographic projection apparatus, and more specifically to a lithographic projection apparatus with a fluid cleaning system.
2. Description of Related Art
The term “patterning device” or “patterning structure” as here employed should be broadly interpreted as referring to a device or structure that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning devices include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing a piezoelectric actuation device. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.        
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper or step and repeat apparatus. In an alternative apparatus—commonly referred to as a step and scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti parallel to this direction; since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion implantation (doping), metallization, oxidation, chemo mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0 07 067250 4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”. However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.
It has been found that used G- and I-line and deep UV microlithography lenses suffer from degradation in terms of loss of overall transmission and loss of wafer illumination uniformity.
In a purged system, i.e., a system that is purged with a purging gas, this degradation is mainly caused by the occurrence of contamination on the surfaces of the first and last optical element in the projection system, i.e., the first optical element encountered by the projection beam and the last optical element encountered by the projection beam in the projection system. It will be understood, however, that in systems that are not purged, crystal growth is likely on other surfaces of the projection system in addition to the surfaces of the first and the last optical elements. Such contamination comprises dendritic salt structures which grow on the lens surfaces. It has been found that lenses subject to intense radiation over a period of time, typically a few years, become contaminated with salt structures. This problem is not limited to the particular type of radiation used, but has been found to occur with radiation of 365 nm, 248 nm, 193 nm, 157 nm as well as extreme ultra violet (EUV) lithography. It is mentioned that EUV lithography apparatus are typically not purged systems. The origin of the lens surface contamination appears to be refractory compounds, such as silane, being present at very low concentrations, i.e., parts per million (ppm) to parts per billion (ppb) in the purge air, which is used as a medium in the lithographic apparatus to stabilize conditions within the apparatus, and have even been found in purified nitrogen used for special purging purposes. Irradiation induced chemical surface reactions of silanes, sulphates or phosphates in combination with the presence of other gases, such as, for example, oxygen, water, and ammonia, are considered to be the basis degradation mechanism. It is believed that nucleation as well as growth of the contaminating crystals occur during exposure with radiation of the G-, I-, deep UV and EUV wavelengths. It is believed that these wavelengths, at least, cause a particular photochemical reaction to occur.
Conventionally, this problem has been addressed by mechanical or chemical cleaning with a non-scratching cloth wetted with specific chemicals. It has been found, however, that this conventional approach results in a spreading or distribution of salt growth nuclei over the entire lens surface. Subsequent use of the lens in the projection system results in accelerated growth of the contamination over the entire “cleaned” lens surface. This effect dramatically reduces the optical throughput, the optical imaging quality and the time between subsequent cleaning. After a number of cleaning rounds, it has been found that removal of the surface contamination becomes more difficult. The occurrence of contamination may finally require a complete interchange of the dirty projection system with a new system, which is very expensive.
The problem of removing contaminants from cooling air is addressed in U.S. Pat. No. 5,696,623, which discloses an air purge system including a method for cleaning cooling gas in a semiconductor manufacturing device. The method includes exposing the cooling air to ultraviolet radiation. One problem with this particular prior art is that it is necessary to cool the air. It has been found that exposing air to ultraviolet prior to it being passed through the lens system does not eliminate salt crystal growth.